NMR is a powerful technique for analyzing molecular structure. However it is also an insensitive technique compared to others for structure determination. To gain maximum sensitivity, NMR magnets and spectrometers are designed to operate at high magnetic field strengths, employ low noise preamplifiers and RF probe coils that operate at cryogenic temperatures using cold normal metal transmit/receive coils or preferably transmit/receive coils made with high temperature superconducting (HTS) materials. The transmit/receive coils are the probe coils that stimulate the nuclei and detect the NMR response from the sample, and therefore are placed very close to the sample to provide high sensitivity. The HTS coils have the highest quality factor, Q, and yield the best sensitivity. Multi-turn spiral coils are commonly used to detect the NMR signal, particularly for lower gamma nuclei such as 13C, 15N and 31P. The electric fields from the turns of the spiral coils near the sample may penetrate the sample and cause dielectric losses and increased noise. The electric fields penetrating the sample also cause detuning of the coil and a resonant frequency that is a function of the dielectric constant and position of the sample. In spinning samples, this detuning can lead to spurious spinning sidebands.
When the RF current flows through the windings of the NMR probe coil, an RF magnetic field is produced in the sample region that stimulates the resonance in the sample. This RF magnetic field, B, has an associated RF electric field, E. This RF electric field, E, can be calculated utilizing the Maxwell equation:curl E=−dB/dt.
To minimize losses from this RF electric field, NMR probes are designed so that the sample is in a region where this RF electric field is a minimum, or passes through zero.
There is another component of electric field that is caused by the electric potential between the windings of an RF coil. This, so-called, conservative electric field arises from the electric potential differences of the turns of the RF coil winding. This component of electric field, Ec, obeys the condition:curl Ec=0.
It is called an electrostatic field since it does not require any time derivatives to produce it. When this component of electric field penetrates the sample or sample tube it can cause energy losses. During transmit and during spin decoupling experiments these losses can cause undesired heating of the sample. During the receive phase, the currents induced by the NMR signal also produce an electric potential between turns of the RF probe coil, causing electric fields to penetrate the sample volume resulting in a loss of Q and reduced sensitivity. Since the sample is usually at or near room temperature and the probe coil is at a very low temperature, noise power is also introduced into the RF probe coil through this electric field coupling. This loss is proportional to the electric field coupling between the sample and the RF probe coil and depends upon the dielectric loss tangent or dissipation factor of the sample and sample tube material and the electrical conductivity of ionic samples.
Small changes in the strength or direction of the DC magnetic field applied to the probe or other magnetic field fluctuations induce small shielding currents in the surface of the superconductor films of the spiral coils. These shielding currents can cause magnetic field inhomogeneity in the sample region resulting in line broadening and loss of NMR sensitivity. (U.S. Pat. No. 5,565,778). To reduce the shielding currents the coil turns may be slit in the direction parallel to that of the RF current flow thereby reducing size of the shielding current loops. Each turn of the spiral coil may be split into a number of parallel conductors or “fingerlets” with a small insulating gap between fingerlets.
Electrostatic shields have been used in the prior art to reduce this electric field. (US Patent Publication No. 2008/0150536). The task of reducing the electric filed in the region of the sample is very important, and therefore there is a need to find an alternative method and apparatus. Existing technology does not utilize the counter-wound spiral coils to minimize the electric field in the region of the sample.
Spiral wound coils have been used before as a surface coil for MRI measurements when it was desired to keep the coil small, and yet be able to tune it to a low frequency (U.S. Pat. No. 5,276,398). It was proposed to use a pair of circular counter-wound coils with capacitive coupling between them for MRI applications to achieve a lower resonant frequency than could be achieved with a single coil. The teaching of this art did not consider or contemplate reducing the electric field over the sample region.
RF coils for high resolution NMR probes must be precisely tuned to the NMR resonant frequency of the nucleus being studied. For maximum sensitivity the electric field from the probe RF coils must produce a minimum electric field in the sample region. The coils are tuned to resonate in the RF frequency range of the nuclear species being studied. The probe is tuned by adjusting the total coil length of wire used to wind the coils. Fine tuning is provided by a variable capacitor or by a wand that provides small adjustable changes to the resonator inductance.
Therefore there is a need in providing the RF probe coils of certain geometries, which are characterized by a strong RF magnetic field with substantially reduced RF electric filed over the sample volume.